The present invention relates to a throw away tool type drill which is constructed so that an end cutting edge is formed by a central cutting edge and a peripheral cutting edge, and to a drill tip. In particular, the present invention relates to a drill which can prevent a breakage of the central cutting edge, and to a drill tip which is formed with a chip breaker.
Conventionally, there is, for example, a throw away tool type drill which has been disclosed in Jpn. Pat. KOKAI Publication No. 6-91416. In this type of drill, a diamond plate-like tip is used, and the tip has the same shape and dimension and is used both as a central cutting edge and a peripheral cutting edge.
Each side of a diamond which functions as a rake face of a tip is formed with a pair of side cutting edges individually adjacent to vertex portions forming an acute angle of the diamond. Also, the vertex portion forming an acute angle is formed with a pair of V-letter corner cutting edges shorter than the side cutting edge. These V-letter corner cutting edges are mutually symmetrical with respect to a bisector of the vertex portion, and form a cross angle lager than an apical angle thereof.
A tip fixing holder is formed with a bottom face and a wall face so that fixing stiffness and accuracy of a tip to a tool body can be enhanced.
Moreover, there is Jpn. UM Appln. KOKOKU Publication No. 7-37769 having a disclosure that a different tip is used for each of a central cutting edge and a peripheral cutting edge.
A tip having a substantially plate-like rectangle is used as the central cutting edge. The tip is formed with an obtuse V-letter cutting edge ridge line in rotation symmetry at its opposite ends in a longitudinal direction thereof. On the other hand, a tip having a plate-like rectangle is used as the peripheral cutting edge. The tip is formed with a linear cutting edge at its opposite ends on each of shorter and longer sides.
In the central cutting edge tip and the peripheral cutting edge tip, their longitudinal direction ends are situated in an axial direction of the tool body, so that a diameter of cutting edge can be made small.
The aforesaid conventional drill has the following problems. Specifically, there are caused a chip breakage in the central cutting edge and a trouble on chip control in the peripheral cutting edge.
In the central cutting, there is case where a breakage is caused in a cutting edge in the vicinity of the center of a drill due to an axial direction pressure caused by feeding motion when a cutting speed becomes zero. In order to avoid such a phenomenon caused in the vicinity of the drill center, the central cutting edge tip has been conventionally arranged in a tool offset state. However, the tool offset with respect to the tip is not sufficient for enhancing a strength of a cutting edge in the vicinity of a rotational center of the drill.
Moreover, the breakage in the central cutting edge results from a factor that a tip corner angle and an inclination angle of a tip cutting edge ridge line with respect to a rotational center line of drill are not properly set. In general, in the case where the tip corner angle and the inclination angle are both large, a cutting edge strength can be enhanced. However, the aforesaid angular setting is not readily varied because of various problems such as a limitation of a tip shape, cutting balance. For this reason, the cutting edge strength could not be enhanced.
On the other hand, a trouble on a chip control is due to a difference in a cutting speed of an end cutting edge on a drill distal end portion. Specifically, in case of viewing the drill distal end portion from an axial direction of the drill, a cutting speed has a proportional relationship with a drill diameter; for this reason, the cutting speed becomes the highest speed in an outer periphery of the drill, and becomes zero in the axis thereof.
Thus, a difference in a cutting speed is great in the vicinity of the axis where cutting by the central cutting edge is executed; for this reason, chips naturally curl up. As a result, there is almost no trouble on chip control.
However, since a difference in a cutting speed is small in an outer periphery side where cutting by the peripheral cutting edge is executed, cut chips are hard to curl up. For this reason, there occurs a case where these chips curl up the tool body. Therefore, the peripheral cutting edge needs to form a chip breaker which is excellent in a chip breakage performance in order to remove these cut chips.
The diamond-shaped tip used in each of the aforesaid Publications is a positive type tip having a chip breaker. However, the tip has no sufficient allowance in its breaker width. For this reason, proper chip control is not widely achieved. Meanwhile, if the breaker width is formed wider, a tip size becomes large; for this reason, it is difficult to make small a drill diameter.
To solve the aforesaid problems, there is required a throw away tool type drill which is capable of collectively satisfying requirements such as breakage prevention of the central cutting edge, improvement in chip control of the peripheral cutting edge, and a scale-down of a drill diameter.